A general method and also CT systems for executing the procedure, are generally known. For example, it is possible—with the help of so-called dual-energy CT systems using two different X-ray radiation spectra—to reconstruct two tomographical image data records of the same examination object, which reflect the respective local attenuation values, or—more precisely—the local attenuation coefficients, that are influenced by the respective radiation spectrum used and by the spectral sensitivity of the detector used.
The measurement data is analyzed mainly by way of basic material decomposition, as described—for example—in PHYS. MED. BIOL., 1976, VOL. 21, NO. 5, 733-744, “Energy-selective Reconstructions in X-ray Computerized Tomography”, R. E. Alvarez and A. Macovski, the entire contents of which are hereby incorporated herein by reference. In this method, two image data records showing the concentrations of basic materials are generated. Iodine and water or bone and water, for example, are used as typical basic materials. According to existing knowledge, a Rhoz projection may also be carried out on the basis of such tomographical image data records, as described in the publication DE 101 43 131 B4 (the entire contents of which are hereby incorporated herein by reference) or in the Journal of Applied Physics, “Density and atomic number measurements with spectral X-ray attenuation method”, B. J. Heismann et al (the entire contents of which are hereby incorporated herein by reference). In this method the local density and atomic number are calculated on the basis of the energy-specific attenuation values that have been reconstructed via the tomographical image data records.
Such spectral methods achieve a material characterization through the calculation of indirect variables, such as the concentration of basic materials or the effective density and atomic number. Thus, for example, water may ordinarily be identified through a basic material composition of 1 part water and 0 parts bone or by an effective density of 1 g/cm3 and an effective atomic number of 7.5.
One alternative is direct identification of body materials. The measured-radiation-independent attenuation function of water μwater(E) can be calculated with a high degree of accuracy. This is also possible for all organic and inorganic materials with known modular mass composition. In the ICRU 46 Report, for example, these attenuation functions are recorded for many body materials.
For direct identification of materials in CT on the basis of the image or attenuation values calculated therein, a number of limiting factors now emerge:                Quantum noise forms a natural static limit for CT. An absorber may accordingly be identified only with a degree of probability, but not, however, with complete certainty. The dose required—at least in the field of medical applications        must be carefully controlled. In positive terms the x-ray doses currently used in the medical field are sufficient for measuring weighted, so-called mean attenuation coefficients μj({right arrow over (r)}), across the energy spectrum used, locally at the location {right arrow over (r)} with an accuracy of approx. 0.3% to 1%. This corresponds to CT values of 3 to 10 HU per 1000 HU. This is, per se, a good statistical basis for using x-ray attenuation data for material identification.        When an object is penetrated by an x-ray the latter is attenuated both by the photo effect and by the Compton effect. The Compton effect causes radiation scatter. The scattered x-ray quanta leave the attenuation path described by tubes and detector pixels. They are potentially measured in different detector channels as an error signal. By using collimators over the detector pixels, this signal proportion is greatly reduced—typically from several tens of percentage points to a few percent.        In generally known methods, beam hardening leads to a local underestimation of attenuation values. The displacements that occur limit the accuracy of quantitative evaluations.        
The identification of materials in an examination object is already being trialed on an image basis in several applications. Examples of this are the representation of uric acid (gout) or cardiac muscle damage. The limits described above with regard to dose, radiation scatter and beam hardening also apply in these cases.
A range of reconstructive correction processes also exists for reducing the effects of radiation scatter as a limiting factor in direct material identification, and these are used—in particular—in dual-source CT, since it is here that the effect of radiation scatter is relatively strong.
The so-called poly-correction process has hitherto been used mainly for correcting beam hardening. The purpose of such beam hardening correction methods is, in particular, to improve the display of the soft tissue gray level. Typical cupping effects or dark regions in the vicinity of bone are largely avoided with these known methods.